Communication Method In An Automatic System

ABSTRACT

A communication method in an automation system, wherein the system comprises an access point and client terminals that receive downlink data sent from the access point and send uplink data to the access point, and the data transmission is divided into a master multiple receiver aggregation phase and more than one slave multiple receiver aggregation phases. During the master multiple receiver aggregation phase, the access point aggregates the data to be sent to the plurality of client terminals into one packet for transmitting in the downlink, and during the slave multiple receiver aggregation phases, the access point aggregates data in which errors have occurred when being transmitted during the master multiple receiver aggregation phase into one packet for transmitting in the downlink. The method can have strict control, through the access point and over the uplink and downlink data transmission, thus enhancing the certainty during automation communication and further reducing the cycle time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/CN2009/071582 filed30 Apr. 2009, the content of which is incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication method in an automationsystem and, more particularly, to a communication method which is usedto perform real-time deterministic communication in an automationsystem.

2. Description of the Related Art

In the series of automation products SIMATIC, the PROFINET IO protocol(PNIO protocol) is used between distributed I/O devices and an I/Ocontroller to perform fast and effective data exchange. The PROFINET IOprotocol introduces the industrial Ethernet technology into the field atfirst level. The PNIO communication is acyclic communication, and inthis communication mechanism, the I/O controller communicates with eachof the I/O devices one by one within a time interval that is determinedin advance. The time interval is referred to as the cycle time or PNIOupdating time, and this time interval must be less than the time cycleof each I/O device that is accessed. In order to ensure that the I/Odevices can be controlled accurately, the above cycle time must be asshort as possible. Therefore, the difficulties faced by such cycliccommunication systems, with the PNIO communication as an example, are:first of all, the initial communication certainty between the I/Ocontroller and the I/O devices needs to be ensured, i.e., the I/Ocontroller is made to communicate with a designated I/O device within adetermined time period. Secondly, the cycle time should number of theI/O devices is increased. In industrial fields, both of the above twopoints are achieved by the PROFINET standards. When the real-timecommunication is performed by using the PROFINET IO, a rather typicalexample is that the cycle time can reach 8 ms in cases where there are60 I/O devices.

Currently, as wireless networks are becoming increasingly popular,wireless LAN (WLAN) devices have also been introduced into theautomation systems, so that another communication connection mode isprovided when cable connections cannot be effectively used with mobiledevices. FIG. 1 provides a schematic diagram of a WLAN communicationusing the PNIO protocol. In this case, an access point (AP) is locatedin the center and the four devices around it are client terminals (STA).

However, in WLAN communication based on the PNIO protocol, thedifficulties faced by the cyclic communication are even more prominent.This is because a distributed coordination function (DCF) is initiallyused in the normal WLAN system to provide a channel access solution.That is, all the client terminals have identical priority to access thechannel at any time. It is apparent that since the data volume to betransmitted by some client terminals may be very large, it is verydifficult for this solution to ensure the certainty and instantaneity ofthe communication. Secondly, it is necessary to introduce a distributedinter frame space (DIFS) and an index backoff solution beforetransmitting the real data to avoid causing collisions in the channelaccess process. However, in these two solutions, in order to ensure thateach device is able to transmit data, a large amount of additionaloverheads are required, and these overheads would then increase thecycle time.

In order to ensure that deterministic communication can be performedbetween the I/O devices and the I/O controller, some channel accesssolutions that are fully or partly controlled by the AP are proposed inthe prior art. For example, a point coordination function (PCF) is alsoproposed together with the above DCF solution in the 802.11 standards.The PCF solution is based on a polling mechanism. That is, the accesspoint (AP) inquires into each client terminal (STA) according to apolling list located thereon. The client terminal (STA) can access thechannel only after having obtained an inquiry from the access point. Forthe data exchange in the PCF solution, reference can be made to FIG. 2.Here, D1 indicates the transfer of downlink data from the access pointAP to a client terminal STA1, D2 indicates the transfer of downlink datafrom the access point AP to a client terminal STA2, and D3 indicates thetransfer of downlink data from the access point AP to a client terminalSTA3. When the access point AP transfers downlink data to the clientterminal STA1, the AP first puts the data D1 to be sent to STA1 togetherwith a poll packet Poll1 to send them to STA1. After having received theabove packet, STA1 first decapsulates the data, which the access pointAP requires to be sent by itself, from the Poll1 packet, then STA1 firstsends an acknowledgement ACK for the downlink data sent by the accesspoint AP, and then sends uplink data to the access point AP. For thereceived uplink data that is sent by STA1, the access point AP sends anacknowledgement ACK. When the downlink data is transferred from theaccess point AP to client terminals STA2 and STA3, this is alsoperformed according to the above steps.

Although the PCF solution has a degree of certainty in communication, itdoes not have complete certainty. In other words, the PCF solution canachieve the control of each client terminal STA through the access pointAP over the downlink, thus achieving real-time and determinatecommunication over the downlink. It is still not possible, however, toensure communication certainty over the uplink. For example, the packetsent by STA1 to the access point AP may be very long, so that the accesspoint AP cannot be certain about the specific time when the packet toSTA2 is sent the next time. In addition, at the moment, the PCF solutionstill does not have effective hardware support. The above two aspects ofdefects are the reasons for the PCF not having widespread application.

Based on the above solutions, a novel multiple receiver aggregation(MRA) solution is proposed to solve the problem of cyclic communication.The MRA solution is achieved by the expansion of the single-receiveraggregation (SRA) solution in the current 802.11n standards. The basicidea of SRA is to have several packets to be sent to a client terminalSTA by the access point AP aggregated together, so as to form a biggerpacket for sending. Two aggregation methods in SRA are respectively theaggregate MAC protocol data unit (A-MPDU) method and the aggregate MACservice data unit (A-MSDU) method. The basic idea of MRA is to have thepackets to be sent to a plurality of client terminals STA1, STA2, andSTA3 by the access point AP aggregated together, so as to form a bigphysical layer convergence protocol (PLOP) service data unit (PSDU),which is referred to as an MRA frame. Before transmitting an MRA frame,it is necessary to initially send a management frame, which is referredto as a multiple receiver aggregation multi-poll frame (MMP). FIG. 3shows the frame format of an MMP.

An MMP frame contains some information indicating how the clientterminals which support MRA receive the packets sent by the access pointAP and send the packets from each client terminal, and the indicationinformation is centralized in a receiver info field in the MMP frame.Specifically, there is an Rx Offset for indicating when the clientterminals which support MRA receive downlink data, a Tx Offset forindicating when the client terminals which support MRA send uplink data,an Rx Duration for indicating the length or the duration of the packetsreceived by the client terminals which support MRA, and a Tx Durationfor indicating the length or the duration of the packets sent by theclient terminals which support MRA.

A typical sequence that uses the MRA solution to perform data exchangeis shown in FIG. 4. In FIG. 4, there is one access point and five clientterminals. The access point AP aggregates the downlink data D1, D2, D3,D4, and D5 to be sent to five (5) client terminals into an MRA frame fortransmitting, and the MRA frame further includes an MMP frame that islocated in the front of the data. In the MMP frame, there are five (5)receiver info fields for indicating respectively how 5 client terminalsreceive the packets sent by the access point AP and send the packets ofeach client terminal. Here, the client terminal STA1 sends anacknowledgement BA of the received data D1 to the access point AP afterhaving received the data D1, the duration of which is Rx Duration,according to the time designated by Rx Offset in the receiverinformation field 1. In addition, the client terminal STA1 further sendsthe data U1, the duration of which is Tx Duration, according to the timedesignated by Tx Offset in the receiver information field 1. The clientterminal STA1 can aggregate the uplink data U1 with its acknowledgementBA of the downlink data D1 together, so as to send them to the accesspoint AP. The access point AP sends out its acknowledgement BA of theuplink data U1 after having received this uplink data U1, and this BA isthe first BA located at the upper right in FIG. 4. It can be seen that,in the MRA solution, both the uplink and the downlink can be strictlycontrolled over the access point AP. In addition, the packets sent toall the client terminals are aggregated. As a result, the timeexpenditure in the downlink communication is reduced and the cycle timeis shortened effectively.

However, there are still problems when the MRA solution is used incommunication. Firstly, the acknowledgement BA for the uplink packettakes a relatively long time. This is because the acknowledgement forthe downlink packet can be achieved by aggregating the acknowledgementinto the uplink packet by way of aggregation or by changing one bit ofthe uplink packet. Accordingly, the acknowledgement for the downlinkpacket does not cost too much time. However, the acknowledgement for theuplink packet cannot be achieved in the way mentioned above. As aresult, the acknowledgement for the uplink packet has to take a certaintime by itself. In this way, in the case where there are quite a fewclient terminals, the access point will take a relatively long time toperform acknowledgement of the uplink data sent by each client terminal.For example, in the case where there are 60 client terminals, the PSDUlength aggregated by all the acknowledgements for the uplink packetwould be 1198 bytes, and assuming that the transmission rate is 6 Mbps,then it would cost a time of more than 1.6 ms to perform acknowledgementof the uplink packet. Secondly, the MRA solution only provides a basicdata transmission sequence but does not give consideration to aretransmission mechanism when errors occur in the packets. Therefore,the application of the MRA solution is limited.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a communicationmethod in an automation system, in which determinate and real-timecommunication can be performed in a practical way in the automationsystem by using the method, so as to have the communication cycle timeshortened as much as possible.

This and other objects and advantages are achieved in accordance withthe present invention by a communication method in an automation systemcomprising an access point and more than one client terminal, in whichclient terminals receive downlink data sent from the access point andsend uplink data to the access point, and the data transmission isdivided into a master multiple receiver aggregation phase and more thanone slave multiple receiver aggregation phases. During the mastermultiple receiver aggregation phase, the access point aggregates thedata to be sent to the more than one client terminals into one packetfor transmitting in the downlink, and during the slave multiple receiveraggregation phases, the access point aggregates the data that haveerrors which happened therein when being transmitted during the mastermultiple receiver aggregation phase into one packet for transmitting inthe downlink.

Preferably, during the slave multiple receiver aggregation phases, theaccess point first aggregates the data that have errors that happenedtherein when being transmitted during a previous phase into one packetfor transmitting in the downlink, and then the client terminals transmitto the access point the uplink data having errors that happened thereinwhen being transmitted during the previous phase. Here, the previousphase is a master multiple receiver aggregation phase or a previous oneof the slave multiple receiver aggregation phases.

In this case, each of the client terminals sends an acknowledgement ofthe received downlink data, together with the uplink data that is to besent thereby, to the access point over the uplink.

In addition, the packet that is sent during the master multiple receiveraggregation phase comprises a master multiple receiver aggregationmulti-poll frame for indicating that the data is transmitted from theaccess point to the more than one client terminals the downlink for thefirst time and indicating that the data is transmitted from the clientterminals to the access point over the uplink for the first time. Thepacket that is sent during the slave multiple receiver aggregation phasecomprises a slave multiple receiver aggregation multi-poll frame forindicating that the data is retransmitted from the access point to morethan one client terminals over the downlink and indicating that the datais retransmitted from the client terminals to the access point over theuplink.

Preferably, the slave multiple receiver aggregation multi-poll framecomprises more than one receiver information fields, with each of thereceiver information fields corresponding to one of the client terminalsand each of the receiver information fields including four fields forrespectively indicating the time when the client terminal receives thedownlink data sent from the access point, the length of the receiveddownlink data, the time when the client terminal sends the uplink datato said access point, and the length of the sent uplink data.

Preferably, the receiver information fields for indicating the time whenthe client terminal sends the uplink data to the access point and thelength of the sent uplink data of the slave multiple receiveraggregation multi-poll frame are set to acknowledge the uplink data sentby the client terminals during the previous phase.

Here, the master multiple receiver aggregation multi-poll frame and theslave multiple receiver aggregation multi-poll frame are defined in areserved domain in the multiple receiver aggregation multi-poll frame.Preferably, the master multiple receiver aggregation multi-poll frameand said slave multiple receiver aggregation multi-poll frame aredefined by two bits in the reserved domain.

The present invention provides a method with stronger certainty for thecommunication between the access point and the client terminals. Firstof all, the present method can not only strictly control the downlinkdata transmission through the access point but also strictly control theuplink data transmission via the access point. Secondly, the method ofthe present invention expands the original MRA solution, dividing amaster-MRA phase and a slave-MRA phase for data retransmission based onthe MRA, which makes the present invention have more practicalapplicability. Furthermore, the present invention further divides theMMP frame into a master-MMP frame and a slave-MMP frame. As compared tothe MMP, the slave-MMP frame replaces the function of performingacknowledgement of the uplink in the MMP, and as compared to the MRAsolution, the cycle time is further shortened.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims. It should be further understood that thedrawings are not necessarily drawn to scale and that, unless otherwiseindicated, they are merely intended to conceptually illustrate thestructures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinbelow, the particular embodiments of the present invention will befurther described in detail in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram of a WLAN communication using the PNIOprotocol in accordance with the prior art;

FIG. 2 is a schematic diagram of data exchange in the PCF solution inaccordance with the prior art;

FIG. 3 is an MMP frame format in accordance with the prior art;

FIG. 4 is a schematic diagram of a typical sequence which uses an MRAsolution to perform data exchange in accordance with the prior art;

FIG. 5 is a schematic diagram of a sequence which uses the master-slaveMRA solution for performing data exchange in according with the presentinvention;

FIG. 6 is a schematic block diagram illustrating modification of theformat of an MMP frame when the master-slave MRA in accordance with thepresent invention;

FIG. 7 is a simulation result, in which the cycle time of the PCF, MRAand master-slave MRA solutions are compared in the case where the BER is10⁻⁵ and the length of the packets is different; and

FIG. 8 is a flowchart of a method in accordance with an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 5 is a schematic diagram of a sequence that uses the master-slaveMRA method of the present invention to perform data exchange. Theautomation communication system in FIG. 5 comprises an access point APand three client terminals STA1, STA2, and STA3. The access point APsends downlink data to the client terminals STA1, STA2, and STA3, andthe client terminals STA1, STA2, and STA3 respectively send uplink datato the access point AP.

In accordance with the method of the present invention, the dataexchange process between the access point AP and three client terminalsSTA1, STA2, and STA3 is divided into two phases, which respectively area master multiple receiver aggregation phase (Master-MRA) and two slavemultiple receiver aggregation phases (Slave-MRAs).

During the master multiple receiver aggregation phase (Master-MRA), theaccess point AP aggregates the data to be sent to client terminals STA1,STA2, and STA3 into one packet, which packet includes a master multiplereceiver aggregation multi-poll frame (Master-MMP). Since there arethree client terminals in this embodiment, the master-MMP also has threereceiver info fields, with each receiver info field defining a datareceiving and sending situation for a client terminal.

FIG. 6 provides the data receiving and sending situation that is definedfor a client terminal STA1 in the reserved field of a receiverinformation field. Here, the master-MMP frame and slave-MMP frame aredefined by using two high bits b3 and b2 of the reserved field. In themaster-MMP, if the client terminal STA1 receives new downlink data fromthe access point AP for the first time, and the client terminal STA1also sends new uplink data to the access point AP for the first time,then b3 and b2 are 00. In the master-MMP, the time and length of thedownlink data D1 sent by the access point AP to client terminal STA1 andthe time and length of the uplink data U1 sent by client terminal STA1to the access point AP are determined by the Rx Offset, Rx Duration, TxOffset, and Tx Duration of the receiver info field for the clientterminal STA1.

Similarly, in the reserved field of the receiver information field forthe client terminals STA2 and STA3, the two high bits b3 and b2 of thereserved field are also 00. In the master-MMP, the time and length ofthe downlink data D2 sent by the access point AP to client terminal STA2and the time and length of the uplink data U2 sent by the clientterminal STA2 to the access point AP are determined by the Rx Offset, RxDuration, Tx Offset, and Tx Duration of the receiver information fieldfor the client terminal STA2. In the master-MMP, the time and length ofthe downlink data D3 sent by the access point AP to the client terminalSTA3 and the time and length of the uplink data U3 sent by the clientterminal STA3 to the access point AP are determined by the Rx Offset, RxDuration, Tx Offset, and Tx Duration of the receiver info field for theclient terminal STA3.

After having received the aggregation packet, the client terminal STA1learns that this MMP is the master-MMP by detecting that b3 and b2 are00 and determines the time and length of the downlink data D1 sent bythe access point AP received thereby and the time and length of theuplink data U1 sent to the access point AP according to the Rx Offset,Rx Duration, Tx Offset, and Tx Duration of the receiver informationfield. Similarly, after having received the aggregation packet, theclient terminal STA2 learns that the MMP is the master-MMP by detectingthat b3 and b2 are 00 and determines the time and length of the downlinkdata D2 sent by the access point AP that is received by the clientterminal STA2 and the time and length of the uplink data U2 sent to theaccess point AP according to the Rx Offset, Rx Duration, Tx Offset, andTx Duration of the receiver information field of the client terminalSTA2. After having received the aggregation packet, the client terminalSTA3 learns that the MMP is the master-MMP by detecting that b3 and b2are 00 and determines the time and length of the downlink data D3 sentby the access point AP that is received by the client terminal STA3 andthe time and length of the uplink data U3 sent to the access point APaccording to the Rx Offset, Rx Duration, Tx Offset, and Tx Duration ofthe receiver info field of the client terminal STA3.

Assuming that errors occur in data D1 and D2 when they are beingtransmitted and data D3 is transmitted correctly, i.e., the clientterminals STA1 and STA2 cannot send out the acknowledgement ACK for thereceived downlink data, only the client terminal STA3 sends out theacknowledgement ACK for the received downlink data. The client terminalSTA3 aggregates the uplink data U3 sent to the access point AP togetherwith the acknowledgement ACK of the downlink data received by the clientterminal STA3 for sending out.

In addition, assume that the uplink data U1 sent by the client terminalSTA1 to the access point AP and the uplink data U3 sent by the clientterminal STA3 to the access point AP are transmitted correctly, and anerror occurs in the uplink data U2 sent by the client terminal STA2 tothe access point AP when it is being transmitted. At this point, themaster multiple receiver aggregation phase (Master-MRA) ends.

The master multiple receiver aggregation phase (Master-MRA) is followedby a first slave multiple receiver aggregation phase (Slave-MRA). Theaccess point AP only receives the acknowledgement ACK for data D3 sentby the client terminal STA3 but does not receive the acknowledgement ACKfor data D1 and data D2 sent by client terminals STA1 and STA2.Consequently, the access point AP can determine that errors occurred inthe downlink data D1 and D2 when they were being transmitted and it isrequired to retransmit data D1 and data D2. In addition, since theaccess point AP only receives the uplink data U1 sent by the clientterminal STA1 and the uplink data U3 sent by the client terminal STA3,but does not receive the uplink data U2 sent by the client terminalSTA2. Consequently, it can be determined that an error has occurred inthe uplink data U2 during transmission and it requires the clientterminal STA2 to retransmit data U2.

The access point AP aggregates the downlink data D1 and D2 to be sent toclient terminals STA1 and STA2 into one packet, which packet includes aslave multiple receiver aggregation multi-poll frame (Slave-MMP), andwhich slave-MMP indicates that the downlink data D1 and D2 areretransmitted from the access point AP to the client terminals STA1 andSTA2 over the downlink and indicates that the uplink data U2 isretransmitted from the client terminal STA2 to the access point AP viathe uplink.

More particularly, during the slave multiple receiver aggregation phase(Slave-MRA), the slave-MMP is still defined by the reserved field(Reserved) of the receiver information field in the MMP frame as shownin FIG. 6. Here, there are three client terminals in the presentlycontemplated embodiment. As a result, the slave-MMP also has threereceiver information fields, with each receiver info field defining adata receiving and sending situation for a client terminal. If theclient terminal STA1 is required to resend the uplink data to the accesspoint AP, then the reserved fields b3 and b2 in the receiver info fieldfor the client terminal STA1 are 01. If the access point AP is requiredto resend the downlink data to the client terminal STA1, then thereserved fields b3 and b2 in the receiver information field for theclient terminal STA1 are 10. Finally, if not only the client terminalSTA1 is required to resend the uplink data to the access point AP butalso the access point AP is required to resend the downlink data to theclient terminal STA1, then the reserved fields b3 and b2 in the receiverinfo field for the client terminal STA1 are 11. The cases of thereserved fields in the receiver information field that are defined forthe second client terminal STA2 and the third client terminal STA3 aresimilar to this.

As mentioned above, since errors have occurred in the downlink data D1and D2 sent by the access point AP to client terminals STA1 and STA2 andan error has occurred in the uplink data U2 sent by STA2 to the accesspoint AP, these data need to be retransmitted. Accordingly, for theclient STA1 detention field, b3 and b2 are set as 10 in the reservedfield for the client terminal STA1, i.e., the client terminal STA1 onlyreceives the retransmitted data D1 but does not send data. In addition,the Tx Offset and Tx Duration in the receiver info field of the clientterminal STA1 can also be set as 0, respectively, to indicate that theaccess point AP acknowledges reception of the uplink data U1, so it doesnot require the client terminal STA1 to retransmit the uplink data U1.That is, setting the Tx Offset and Tx Duration fields in the receiverinformation field of the slave-MMP achieves the acknowledgement of theuplink data U1 transmitted by the client terminal STA1 in the slave-MMP,and it is not required to take additional time to acknowledge the uplinkdata, and thus the cycle time is reduced. In the reserved field for theclient terminal STA2, b3 and b2 are set as 11, i.e., the client terminalSTA2 not only receives the retransmitted data D2 but also sends theretransmitted data U2. Similarly, the Tx Offset and Tx Duration in thereceiver information field of the client terminal STA3 can also be setas 0, respectively, to indicate that the access point AP acknowledgesthe reception the uplink data U3.

During the first slave multiple receiver aggregation phase (Slave-MRA),after having received the aggregation packet sent by the access pointAP, the client terminal STA1 learns that the MMP is a slave-MMP bydetecting that b3 and b2 in the reserved field Reserved of the receiverinformation field of STA1 in the slave-MMP frame are 10, needs tore-receive the downlink data D1, and determines the time and length ofthe downlink data D1 sent by the access point AP received therebyaccording to the Rx Offset and Rx Duration in the receiver info field ofSTA1.

Similarly, after having received the aggregation packet sent by theaccess point AP, the client terminal STA2 learns that the MMP is aslave-MMP by detecting that b3 and b2 in the reserved field Reserved ofthe receiver information field of STA2 in the slave-MMP frame are 11,needs to receive the downlink data D2 again and resend the uplink dataU2, and determines the time and length of the downlink data D2 sent bythe access point AP received thereby and the time and length of theuplink data U2 sent to the access point AP according to the Rx Offset,Rx Duration, Tx Offset, and Tx Duration of the receiver info field ofSTA2.

Assuming that data D1 and D2 are retransmitted correctly, then clientterminals STA1 and STA2, respectively, send the acknowledgements ACK forthe received downlink data D1 and D2 and send these two acknowledgementsACK to the access point AP. In addition, assume that an error stilloccurs the uplink data U2 sent by the client terminal STA2 to the accesspoint AP during retransmission. At this point, the first slave multiplereceiver aggregation phase (Slave-MRA) ends.

The first slave multiple receiver aggregation phase (Slave-MRA) isfollowed by a second slave multiple receiver aggregation phase(Slave-MRA). Here, the access point AP receives the acknowledgement ACKof data D3, sent by client terminals STA1 and STA2. As a result, theaccess point AP can determine that the downlink data D1 and D2 have beenretransmitted correctly. In addition, since the access point AP does notreceive the uplink data U2 sent by the client terminal STA2, it can bedetermined that an error has occurred in the uplink data U2 duringretransmission, and it requires the client terminal STA2 to retransmitdata U2.

In the reserved field for the client terminal STA2, b3 and b2 are set as01, i.e., the client terminal STA2 only sends the retransmitted data U2and does not need to receive the retransmitted data.

During the second slave multiple receiver aggregation phase (Slave-MRA),after having received the packet including the slave-MMP sent by theaccess point AP, the client terminal STA2 learns that the MMP is aslave-MMP by detecting that b3 and b2 in the reserved field Reserved ofthe receiver information field of STA2 in the slave-MMP frame are 01,needs to resend the retransmitted data U2, and determines the time andlength of the uplink data U2 resent thereby according to the Tx Offsetand Tx Duration in the receiver information field of STA2. At thispoint, the second slave multiple receiver aggregation phase (Slave-MRA)ends.

In order to prove the effectiveness of the master-slave MRA method inaccordance with the contemplated embodiments of the present invention,simulation of this method is performed and it is compared with the PCFand MRA methods. Here, the simulation was performed based on a simulatorns2. Here, is assumed that the I/O controller communicates with 32 I/Odevices, i.e., one access point and 32 client terminals. This simulationis based on the following assumptions: only the packet error rate of thedata frames is taken into account; since the packet error rate of thecontrol/management frame is very small, it is not taken into account;the uplink and downlink use the same packet length; in order to obtainthe maximum cycle time required, the number of times of retransmissionis not limited; the GreenField mode in 802.11n is used during the wholesimulation process; and the transmission rate of downlink data is 130Mbps and there are 2 spatial streams, while the transmission rate ofuplink data is 65 Mbps and there is 1 spatial stream.

FIG. 7 is a simulation result, in which the cycle times of the PCF, MRA,and master-slave MRA solutions are compared in the case where BER is10⁻⁵ and the packet lengths are different. In FIG. 7, the horizontalaxis represents the cycle time, with the unit being milliseconds ms. Thevertical axis represents the number of the times that appear during onecertain cycle time period. The uppermost diagram in FIG. 7 is thesimulation result when the PCF solution is used and the packet lengthsare different; the middle diagram is the simulation result when the MRAsolution is used and the packet lengths are different; and the lowermostdiagram is the simulation result when the master-slave MRA solution isused and the packet lengths are different. In this case, right slash,left slash, blank line, and grid line in each of the figures representthe simulation results when the packet lengths are 10 B, 64 B, 500 B,and 1000 B, respectively. It can be easily seen from FIG. 7 thatfirstly,no matter how long the packet lengths are, the master-slave MRA solutionin accordance with the presently master-slave MRA solution in accordancewith the presently contemplated embodiments of the invention always hasthe shortest cycle time. Secondly, with the increase of the packetlength, the cycle time becomes more and more dispersed in all threesolutions PCF, MRA, and master-slave MRA. However, the time interval ofthe cycle time for each solution is relatively stable, for example, theinterval between the master-slave MRA and the PCF in this simulation is3.5 ms. Furthermore, with the increase of the packet length, the cycletimes in the three solutions PCF, MRA and master-slave MRA are allincreased, which is due to the increase of the retransmission timerequired.

What are described above are merely the preferred embodiments of thepresent invention, and it should be pointed out that those skilled inthe art can make some improvements and modifications without departingfrom the principles of the present invention, and these improvements andmodifications should also be viewed as within the protection scope ofthe present invention.

FIG. 8 is a flow chart of a communication method in an automation systemin accordance with an embodiment of the invention, where the systemcomprising an access point and a plurality of client terminals receivingdownlink data sent from the access point and sending uplink data to theaccess point, The method comprises dividing the data transmission into amaster multiple receiver aggregation phase and a plurality of slavemultiple receiver aggregation phases, as indicated in step 810.

The access point aggregates the data to be sent to the plurality ofclient terminals into a single packet for transmittal in the downlinkduring the master multiple receiver aggregation phase, as indicated instep 820. The access point aggregates, during the plurality of slavemultiple receiver aggregation phases, data in which errors have occurredwhen being transmitted during the master multiple receiver aggregationphase into the single packet for the transmittal in the downlink, asindicated in step 830.

Thus, while there have shown and described and pointed out fundamentalnovel features of the invention as applied to a preferred embodimentthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the devices illustrated, and intheir operation, may be made by those skilled in the art withoutdeparting from the spirit of the invention. For example, it is expresslyintended that all combinations of those elements and/or method stepswhich perform substantially the same function in substantially the sameway to achieve the same results are within the scope of the invention.Moreover, it should be recognized that structures and/or elements and/ormethod steps shown and/or described in connection with any disclosedform or embodiment of the invention may be incorporated in any otherdisclosed or described or suggested form or embodiment as a generalmatter of design choice. It is the intention, therefore, to be limitedonly as indicated by the scope of the claims appended hereto.

1.-10. (canceled)
 11. A communication method in an automation system,said system comprising an access point and a plurality of clientterminals receiving downlink data sent from the access point and sendinguplink data to the access point, comprising: dividing the datatransmission into a master multiple receiver aggregation phase and aplurality of slave multiple receiver aggregation phases; aggregating, bythe access point, the data to be sent to the plurality of clientterminals into a single packet for transmittal in the downlink duringthe master multiple receiver aggregation phase; and aggregating, by theaccess point, during the plurality of slave multiple receiveraggregation phases, data in which errors have occurred when beingtransmitted during the master multiple receiver aggregation phase intothe single packet for the transmittal in the downlink.
 12. Thecommunication method in an automation system as claimed in claim 11,wherein during the plurality of slave multiple receiver aggregationphases, the access point initially aggregates the data in which errorshave occurred when being transmitted during a previous phase into thesingle packet for transmittal in the downlink, and the plurality ofclient terminals transmit to the access point uplink data in whicherrors have occurred when being transmitted during the previous phase.13. The communication method in an automation system as claimed in claim12, wherein the previous phase is one of the master multiple receiveraggregation phase and a previous slave multiple receiver aggregationphase of the plurality of slave multiple receiver aggregation phases.14. The communication method in an automation system as claimed in claim11, wherein each of said plural client terminals sends over the uplinkto said access point an acknowledgement of downlink data receivedthereby together with uplink data which are to be sent thereby.
 15. Thecommunication method in an automation system as claimed in claim 12,wherein each of said plural client terminals sends over the uplink tosaid access point an acknowledgement of downlink data received therebytogether with uplink data which are to be sent thereby.
 16. Thecommunication method in an automation system as claimed in claim 11,wherein the single packet sent during the master multiple receiveraggregation phase comprises a master multiple receiver aggregationmulti-poll frame for indicating that the data is initially transmittedfrom the access point to the plurality of client terminals over thedownlink for a first time and for indicating that the data is initiallytransmitted from the plurality of client terminals to the access pointover the uplink.
 17. The communication method in an automation system asclaimed in claim 11, wherein the single packet sent during each of saidplural slave multiple receiver aggregation phases comprises a slavemultiple receiver aggregation multi-poll frame for indicating that thedata is retransmitted from the access point to the plurality of clientterminals over the downlink and for indicating that the data isretransmitted from the plurality of client terminals to the access pointover the uplink.
 18. The communication method in an automation system asclaimed in claim 17, wherein the slave multiple receiver aggregationmulti-poll frame comprises a plurality of receiver information fields,each said plural receiver information fields corresponding to arespective client terminal of the plurality of client terminals and eachof said plural receiver information fields comprising four fields forrespectively indicating a time when the respective client terminal ofthe plurality of client terminals receives the downlink data sent fromthe access point, a length of the downlink data received, a time whenthe client terminal of the plurality of client terminals sends theuplink data to the access point, and a length of the sent uplink data.19. The communication method in an automation system as claimed in claim18, wherein a receiver information field of the plurality of receiverinformation fields for indicating the time when the respective clientterminal of the plurality of client terminals sends the uplink data tothe access point and the length of the uplink data sent in said slavemultiple receiver aggregation multi-poll frame are set to acknowledgethe uplink data sent by the client terminal of the plurality of clientterminals in a previous phase.
 20. The communication method in anautomation system as claimed in claim 16, wherein the single packet sentduring each of said plural slave multiple receiver aggregation phasescomprises a slave multiple receiver aggregation multi-poll frame forindicating that the data is retransmitted from the access point to theplurality of client terminals over the downlink and for indicating thatthe data is retransmitted from the plurality of client terminals to theaccess point over the uplink, and wherein the master multiple receiveraggregation multi-poll frame and the slave multiple receiver aggregationmulti-poll frame are defined in a reserved domain of a multiple receiveraggregation multi-poll frame.
 21. The communication method in anautomation system as claimed in claim 20, wherein the master multiplereceiver aggregation multi-poll frame and a slave multiple receiveraggregation multi-poll frame are defined by two bits in a reserveddomain of a multiple receiver aggregation multi-poll frame.